1. Field of the Invention
The present invention relates to a method for producing a glass preform for an optical fiber. More particularly, it relates to a method for producing a glass preform from silicon compounds by means of a multi-nozzle burner.
2. Description of the Prior Arts
The vapor phase axial deposition method (hereinafter referred to as "VAD" method) generally comprises jetting, from a burner, a mixture of a fuel gas, a glass-forming raw material and optionally an additive for adjusting a refractive index of the produced glass preform, flame hydrolyzing the glass-forming raw material to synthesize glass soot particles, depositing the glass soot particles on a rotating starting member to form a porous soot preform and then heating and sintering the porous preform to obtain a transparent glass preform.
In the VAD method, preferably, the fuel gas comprises a mixture of hydrogen and oxygen, the glass-forming raw material comprises SiCl.sub.4, and the additive comprises GeCl.sub.4, POCl.sub.3 or BBr.sub.3. To stably produce the large porous soot preform, a bulk density distribution in the soot preform should be adequately adjusted. In the production of the large porous soot preform, difficulties to be overcome are that the periphery of the preform tends to crack due to small bulk density at the periphery, and that since the bulk density distribution has peculiar points, residual stress is generated in the preform, which causes cracking or prevents steady growth of the soot preform.
The bulk density distribution in the porous soot preform varies with a temperature distribution in a stream of the glass soot particles in the flame and influenced by spread of the flame generated by the burner. When the temperature of the glass soot particles is not high enough or when a whole surface of the deposited particles is not heated, the periphery of the soot proform tends to consist of glass soot particles with low bulk density, which causes cracking.
When SiCl.sub.4 alone is used as the glass-forming raw material, the glass soot particles are synthesized according to the following equation: EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl (I)
Although this reaction is exothermic, it should be promoted by heat evolved by the oxyhydrogen flame since its reaction heat is as small as 24 kcal/mol. Thus, the synthesizing rate of the glass soot particles depends on quantity of heat conducted from the flame and, consequently, the temperature distribution in the stream of the fine glass soot particles depends on the heat of the flame. In order to heat the stream of glass soot particles at a sufficiently high temperature, it is necessary to narrow the spread of the flame and to raise a temperature of the flame center. However, if the spread of the flame is narrowed, the whole surface of the deposited glass soot particles is not heated. This is one of the causes of cracking of the large porous soot preform. As understood from the above discussions, not only the whole surface of the deposited glass soot particles should be heated but also the glass soot particles should be heated at a sufficiently high temperature in order to stably produce the large porous soot perform.